1. Field of the Invention
The present invention is directed generally to systems for increasing the transient stability of power supplied to subsurface pumps, and particularly to the use of pulse width modulation to increase transient stability.
2. Description of the Prior Art
The present invention is a control system for a variable speed controller which may be utilized in conjunction with subsurface pumps, such as electrical submersible pumps, in order to supply power to the subsurface pump, but to do so in a way that allows for protection of the subsurface pump from positive transient power surges which represent a potentially detrimental operating condition for the electrical submersible pump, cables, and associated equipment. The relevant prior art will now be discussed.
In the prior art, two basic variable speed controller control systems are utilized to provide power to a subsurface pump which can be controlled by the operator in both its frequency and amplitude. As is conventional, sinusoidal power is supplied as an input to a controller. The controller rectifies the sinusoidal input to provide a DC power supply. Switching circuits are utilized to switch positive and negative DC voltage in a manner which in the aggregate simulates a sinusoidal output with a particular amplitude and frequency.
The frequency and amplitude of the simulated AC output may be controlled utilizing numerous algorithms. The most common algorithms are a six-step process or a pulse width modulation process. In six-step control, a sinusoidal waveform of a particular frequency and amplitude is constructed from a combination of six segments (three for each half cycle) with each segment having a particular fixed and uniform duration to establish the waveform's overall frequency, and the amplitude being controlled by varying the DC bus voltage by means of a controlled converter (or rectifier). In contrast, in pulse width modulation, the converter is controlled so that the DC bus voltage is constant and the sinusoidal waveform is constructed from combinations of positive pulses and negative pulses, whose width vary in proportion to the instantaneous value of the ideal waveform at each instant to control the amplitude.
Conventional pulse width modulation is disfavored in the industry since a continuous pulse width modulation represents a greater number of opportunities for the generation of high peak voltages, due to the greater number of pulses typically utilized to generate a sinusoidal waveform. High peak voltages, such as positive transients, are especially harmful to subsurface electrical systems such as electrical submersible pumps and should be avoided. High peak voltages present a threat to cable and insulation within the subsurface pump. Additionally, high, uncontrolled peak voltages represent a risk of setting up resonance within the subsurface electrical system, since the subsurface components represent various capacitive, inductive, and resistive values which could be pushed into resonance. If a transient of unknown amplitude and frequency acts upon the system, there is some chance that resonance will be established which can be detrimental to the wellbore pump.
Increasing the amount of control that one has over subsurface equipment is one broad industry goal. Additionally, to increase such control without introducing any additional risks of malfunction is also an important industry goal.